What regulations are in place, or necessary for importing used automotive engines and transmissions?

As a mechanic in a busy automotive shop in berkeley, we frequently do engine, or transmission replacements. Sometimes, to give customers a more economical option on a high mileage vehicle with engine damage, our shop will purchase "used low mileage" engines to replace them with. We them replace alot of the parts on these engines, before installing them back in the car. What are the regulations in place to these Japanese engine Importers that imports these products from Japan? Should we be worried about the engines arriving in the near future?